In the electronics industry, solders are used for assembling electronic circuit boards. To function effectively, solder must wet the substrate with a strong, permanent bond to be formed. Often, a flux is used to enhance the task of wetting the substrate. The ability to remove the surface oxide films from the substrate is another desirable characteristic of a flux. The constituents of most fluxes are therefore often both corrosive and ionically conductive.
Solder joints form interconnections between various levels of an electronic package. These joints are made between solderable metallized surfaces, such as Cu, Cu plated with Pb-Sn, Ni, or Ni plated with Au. The metallized layers are typically heavily contaminated with metal oxides, carbon compounds, and other materials due to extended exposure in the manufacturing environment. A metal surface contaminated by these materials cannot be wet by solder. However, once this surface contamination is removed, the solder wets the metallization and forms a metallurgically sound solder joint, which will both hold the various electronic components in place and pass electrical signals.
Historically, oxides have been removed from metallized surfaces using solder processes by the application of liquid fluxes. Flux (from the latin term meaning "to flow") is applied to a surface to assist in wetting by the solder. Conventionally, a flux consists of active agents dissolved or dispensed in a liquid carrier. The carrier for flux is typically alcohol-based, with varying concentrations of acids or salts as activators. The function of the activators is to reduce base metal oxides. The activators most commonly used in fluxes are: abietic acid (also present in rosin), adipic acid, and amine salts.
The purpose of a flux is to:
(1) remove the oxide from the metallization;
(2) remove the oxide on the molten solder to reduce the surface tension and enhance flow;
(3) inhibit subsequent oxidation of the clean metal surfaces during soldering; and
(4) assist in the transfer of heat to the joint during soldering.
After a solder joint is formed, a flux residue remains. The residue consists of a carrier, such as rosin or resin that is not evaporated, acid or salt deposits, and the removed oxides. The residue can be deleterious to the long-term reliability of an electronic package if it is not removed. The resin can absorb water and become an ionic conductor which could result in electrical shorting and corrosion. The residual activator can, over a period of time, corrode the soldered components and cause electrical opens. Furthermore, the flux residue, which still covers the entire assembly, is present not only on the surface, but also underneath the components where inspection is difficult and the residue is hard to remove.
The current practice is to either select a no-clean flux or use post-soldering cleaning to remove the flux residue.
The use of fluxes which leave a benign flux residue can avoid cleaning after soldering. However, most of no-clean fluxes available today contain a very high percentage of volatile organic compounds (VOCs), such as isopropanol, which are emitted to the environment during the soldering process. VOCs are harmful to the environment and are regulated by the EPA and many states.
If fluxes which leave corrosive and/or hygroscopic residues are used, post soldering cleaning using chlorinated fluorocarbons (CFCs), organic solvents, semi-aqueous solutions, or water is required. For this type of process, in addition to VOC emissions from the soldering process, the cleaning process results in emission of CFCs and waste water. All add to environmental pollution and production costs.
It is known that microorganisms (microbes, e.g. bacterial, fungi, and molds) may grow in aqueous-based fluxes. The microbes most likely are spawned from inadvertent contamination by sources in the environment: dust and dirt particles, for example, carry common soil bacteria.
Microbial contamination and fluxes could, under some circumstances, adversely affect the soldering process. For example, microbial growth consumes adipic acid, converting it to a biomass, CO.sub.2, and possibly partially oxidized products of adipic acid. Accordingly, there is a concern that solderability could be inhibited by contaminated flux forming a coating of biomass or biomaterials such as protein before contact with a solder wave. Additionally, post-solder microbial residues could impact electrical reliability and conformal coat adhesion. Further, microbial consumption of adipic acid could deactivate the flux.
One approach to ameliorating this problem is the use of certain biocidal chemicals. However, many such chemicals known today tend to have a very low human exposure limit and result in ionic or halide residues.
One approach is suggested by U.S. Pat. No. 5,085,365. That reference discloses a non-toxic, noncorrosive liquid soldering flux comprising an organic acid and a non-toxic carrier, such as water. The flux requires cleaning after soldering with deionized water, or other solvents, which generates waste requiring disposal. U.S. Pat. No. 4,360,392 discloses a solder flux composition having as a solvent a mixture of water and one or more aliphatical alcohols. U.S. Pat. No. 2,581,820 discloses the use of distilled water in a soldering flux. U.S. Pat. No. 4,994,119 discloses a water-soluble soldering flux containing deionized water as the solvent. U.S. Pat. No. 3,925,112 discloses that a bacteriostatic agent may be added to a water-based flux.